Conventionally, as an LED element formed of a nitride semiconductor, a semiconductor layer structure is formed on a sapphire substrate (laminated semiconductor substrate) as typified by a blue LED. Such a technique is disclosed, for example, in the following Patent Document 1 and Patent Document 2.
Patent Document 1 discloses an LED having such a structure that on top of a sapphire substrate, an n-type contact layer formed of n-GaN, an n-type cladding layer formed of n-AlGaN, an active layer formed of n-InGaN, a p-type cladding layer formed of p-AlGaN, and a p-type contact layer formed of p-GaN are laminated in sequence. The active layer is realized by a single quantum well structure or a multi-quantum well structure.
Between the sapphire substrate and the n-type contact layer, a buffer layer formed of GaN, AlGaN or MN is formed. The n-InGaN that forms the active layer is doped with a donor impurity such as Si or Ge and/or an acceptor impurity such as Zn or Mg.
Patent Document 2 discloses the feature of growing, on AlN of which plane direction is aligned to the c-axis direction, a GaN layer having a lattice constant larger than that of the AlN and having a plane direction aligned to the c-axis direction, and forming, on the GaN layer, an n-AlGaN layer having a lattice constant smaller than that of the GaN, an active layer having a multi-quantum well structure, and a p-AlGaN layer in this sequence in a laminated semiconductor substrate that forms an LED.